1. Field of the Invention
The invention relates to an electric heating device, in particular to an electric heating device for a motor vehicle.
2. Description of the Related Art
The interior of a motor vehicle is usually heated by means of the cooling water of the vehicle drive unit. The heat which is generated by the internal combustion engine and discharged to the cooling water is subsequently released, upon request, to the air flowing into the vehicle interior. However, this heat is only available after a certain period of operation of the motor vehicle, such that during colder days, the responsiveness of the heating system is inadequate. Especially with the development of new engines which generate less heat energy to be dissipated via the cooling water, a smaller amount of heat energy is available for heating the vehicle. As a result, the amount of heat energy is in no way sufficient for heating the vehicle in the event of extremely low outside temperatures. Even for normal heating, the heating system provides comfortable interior temperatures only after a considerable time delay.
For the above reasons, additional heating systems have already been proposed. These systems are introduced into the air-channeling ducts to discharge heat to the air flowing into the vehicle interior. The arrangement of an additional heating system between a heat exchanger of a vehicle heating system and the vehicle compartment is described, for example, in the essay written by Burk, Kraus, Loble "Integrales Klimasystem fur Elektroautomobile" [Integral air-conditioning system for electric vehicles] ATZ Autombiltechnische Zeitschrift, Issue 11, 1992.
EP-A-A 0 243 077 discloses an additional electric heating system for a motor vehicle, in the case of which, PTC (Positive Temperature Coefficient) elements are in heat-conductive connection with corrugation ribs, and the heat generated by the PTC elements is discharged via the corrugation ribs to the air flowing through an air-channeling duct. The entire arrangement comprises two metal plates, the PTC elements between them, and corrugation ribs arranged on the other side of the metal plate, and has a thin insulation material wrapped around it. The insulation material simultaneously forms the air-channeling duct. Due to the cross section available, only a low heating output can be transferred in the case of such an arrangement, so that the requirements concerning safety (defrosting of the windscreen) and comfort are not realized.
In addition, EP-B-0 350 528 discloses a radiator which comprises a plurality of heating elements which are arranged in parallel, are assembled to form a block and are fastened in a frame. PTC elements are retained between two metal plates. Located respectively on the other side of the metal plates is a corrugation rib which is in heat-conductive connection with the metal plates.
In this arrangement, each heating element is provided with an electrical connection which is guided through a frame part to the outer side of the radiator. For radiators with a greater heating output and which thus require a greater number of heating elements, the number of necessary electrical connections and the requisite insulations in the region of the lead-throughs is also multiplied. Since the corrugation rib is connected integrally to the metal plates adjacent thereto, the heating elements cannot be prefabricated since they are formed only when the entire radiator has been produced, and only by inserting the PTC elements between the metal plates provided with corrugation ribs. It is thus only possible to check the heating element when the radiator is completed. In other words, the entire radiator has to be removed if there is a fault in a heating element.